Blast-resistant reinforced cementitious panels and reinforcing structures for use therein

ABSTRACT

Blast-resistant reinforced cementitious panels are disclosed. The panels include reinforcing structures including wire grid assemblies and connecting wires. The reinforcing structures may be pre-formed prior to the addition of the cementitious material to the reinforcing structure.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/722,504 filed Nov. 5, 2012, which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to blast-resistant reinforced cementitious panels, and more particularly relates to concrete and other types of cementitious materials containing reinforcement structures that provide improved blast performance.

BACKGROUND INFORMATION

Various attempts have been made to reinforce concrete building panels in order to withstand explosive forces. However, a need exists for panels having greater blast resistance, particularly when the blasts are of relatively long duration, for example, as experienced during petrochemical explosions.

SUMMARY OF THE INVENTION

The present invention provides reinforcing structures for concrete and other cementitious material panels that provide improved blast performance. The reinforcing structures may be pre-formed prior to the addition of the cementitious material to the reinforcing structure. The blast-resistant cementitious panels may be pre-cast, cast-in-place or tilt-up panels.

An aspect of the present invention is to provide a reinforcing structure for a blast-resistant cementitious panel comprising a first wire grid assembly comprising an exterior wire grid and an interior wire grid spaced from the exterior wire grid, a second wire grid assembly spaced from the first wire grid assembly comprising at least an exterior wire grid, and a plurality of connecting wires attached to the wire grids of the first and second wire grid assemblies.

Another aspect of the present invention is to provide a reinforced blast-resistant cementitious panel comprising a cementitious material, and a reinforcing structure in the cementitious material. The reinforcing structure comprises a first wire grid assembly comprising an exterior wire grid and an interior wire grid spaced from the exterior wire grid, a second wire grid assembly spaced from the first wire grid assembly comprising at least an exterior wire grid, and a plurality of connecting wires attached to the wire grids of the first and second wire grid assemblies.

A further aspect of the present invention is to provide a method of making a reinforced blast-resistant cementitious panel comprising introducing a flowable cementitious material to a pre-formed reinforcing structure and allowing the cementitious material to cure, wherein the reinforcing structure comprises a first wire grid assembly comprising an exterior wire grid and an interior wire grid spaced from the exterior wire grid, a second wire grid assembly spaced from the first wire grid assembly comprising at least an exterior wire grid, and a plurality of connecting wires attached to the wire grids of the first and second wire grid assemblies.

These and other aspects of the present invention will be more apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of a pre-formed reinforcing structure for a blast-resistant cementitious panel in accordance with an embodiment of the present invention.

FIG. 2 is a partially schematic end sectional view of a reinforced blast-resistant concrete panel in accordance with an embodiment of the present invention.

FIG. 3 is a partially schematic end sectional view of a reinforced blast-resistant concrete panel including an interior foam layer in accordance with another embodiment of the present invention.

FIG. 4 is a partially schematic front view of a wire grid of a reinforcing structure in accordance with an embodiment of the present invention.

FIGS. 5A-5C are photographs illustrating reinforced panels that have been subjected to blast testing.

FIGS. 6A-6C are photographs illustrating reinforced panels that have been subjected to blast testing.

FIGS. 7A-7F are photographs illustrating reinforced panels that have been subjected to blast testing.

FIGS. 8A-8C are photographs illustrating reinforced panels that have been subjected to blast testing.

FIGS. 9A-9C are partially schematic side views of panels illustrating blast deformation characteristics.

FIGS. 10A and 10B illustrate crack patterns for panels subjected to blast testing.

FIGS. 11A-11F are graphs of deflection versus time and bottom support reaction versus time for panels subjected to blast testing.

DETAILED DESCRIPTION

FIGS. 1-4 illustrate reinforced cementitious panels and reinforcing structures for use therein in accordance with embodiments of the present invention. FIG. 2 is an end sectional view showing a reinforced blast-resistant cementitious panel 10 including a cementitious material 12 with a reinforcing structure 20 embedded therein. The multiple arrows above the reinforced panel 10 in FIG. 2 represent the force of an explosive blast that the panel could be subjected to. FIG. 1 is a perspective view of the reinforcing structure 20. As shown most clearly in FIG. 2, the reinforcing structure 20 includes a first wire grid assembly 21 near one exterior face of the cementitious material 12, and a second wire grid assembly 25 near the opposite exterior face of the cementitious material 12.

The first wire grid assembly 21 includes an exterior wire grid 22 and an interior wire grid 23. The exterior and interior wire grids 22 and 23 are located relatively close to each other, and lie in planes that are substantially parallel with each other. FIG. 4 is a front view illustrating a portion of the exterior wire grid 22, which includes multiple horizontal wires 22 a and multiple vertical wires 22 b. The multiple vertical wires 22 b are also shown in the sectional end view of FIG. 2.

As shown in FIG. 2, the second wire grid assembly 25 includes an exterior wire grid 26 and an interior wire grid 27. Each of the exterior and interior wire grids 26 and 27 of the second wire grid assembly 25, as well as the interior wire grid 23 of the first wire grid assembly, may be of the same or similar construction as the exterior wire grid 22 of the first wire grid assembly 21, as shown in FIG. 4, i.e., each wire grid 23, 26 and 27 may comprise multiple horizontal wires and multiple vertical wires.

As further shown in FIG. 2, the reinforcing structure 20 includes multiple connecting wires 30 extending between, and connected to, the first and second wire grid assemblies 21 and 25. In the embodiment shown, each of the connecting wires 30 are attached to the exterior and interior wire grids 22 and 23 of the first wire grid assembly 21, as well as the exterior and interior wire grids 26 and 27 of the second wire grid assembly 25. Alternatively, the connecting wires 30 may only be connected to the interior wire grids 22 and 27, or the exterior wire grids 23 and 26. For example, each of the first and second wire grid assemblies 21 and 25 may be pre-formed by first attaching their respective exterior and interior wire grids together by connecting wires or rods (not shown), followed by attaching the first and second wire grid assemblies together with the connecting wires 30.

In certain embodiments, the connecting wires 30 are attached to the various wire grids by welding. For example, the connecting wires 30, exterior and interior wires 22 and 23 of the first wire grid assembly 21, and exterior and interior wires 26 and 27 of the second wire grid assembly 25 may be made of steel that is electro-welded at some or all of the points of contact between the various wires.

As shown in FIGS. 1 and 2, the reinforcing structure 20 also includes multiple reinforcing rods 42 and 44. The first set of reinforcing rods 42 are engaged with the first wire grid assembly 21 by placing the reinforcing rods 42 in the space between the exterior and interior wire grids 22 and 23. The second set of reinforcing rods 44 are engaged with the second wire grid assembly 25 by placing the reinforcing rods 44 in the space between the exterior and interior wire grids 26 and 27. The reinforcing rods 42 and 44 may be held in place by frictional engagement or contact with the wires of their respective first and second wire grid assemblies 21 and 25. Alternatively, the reinforcing rods 42 and 44 may be attached within the first and second wire grid assemblies 21 and 25 by any other suitable means such as welding, mechanical fasteners, wire ties, fabric ties, or the like.

In accordance with embodiments of the present invention, the configurations, sizes and spacings of the various components of the reinforcing structure 20 are selected in order to produce desirable blast-resistance performance when the reinforcing structure 20 is used in a cementitious panel. In certain embodiments, the wires of the first and second wire grid assemblies 21 and 25 may have typical diameters of from 1 to 7 mm, for example, from 1.5 to 6 mm, or from 2 to 5 mm. In certain embodiments, the connecting wires 30 may have typical diameters of from 1 to 7 mm, for example, from 1.5 to 6 mm, or from 2 to 5 mm. In certain embodiments, the reinforcing rods 42 and 44 may have typical diameters of from 8 to 40 mm, for example, from 10 to 30 mm, or from 12.5 to 25 mm.

As shown in the embodiment of FIG. 2, the first and second wire grid assemblies 21 and 25 may be spaced apart a distance S_(A) that may typically range from 1 to 20 cm, for example, from 4 to 18 cm, or from 7 to 15 cm. In the first wire grid assembly 21, the exterior and interior wire grids 22 and 23 may be spaced apart a distance S_(G), which may typically range from 8 to 40 mm, for example, from 10 to 30 mm, or from 12.5 to 25 mm. Similarly, in the second wire grid assembly 25, the exterior and interior wire grids 26 and 27 may be spaced apart the same distance S_(G) as described above for the first wire grid assembly 21. Although the grid spacings S_(G) shown in FIG. 2 are the same for both the first wire grid assembly 21 and second wire grid assembly 25, it is to be understood that different grid spacings S_(G) may be used if desired.

As shown in FIG. 4, the exterior wire grid 22 of the first wire grid assembly 21 includes multiple horizontal wires 22 a that are spaced apart a distance of D_(H). The horizontal wire spacing D_(H) may typically range from 20 to 200 mm, for example, from 30 to 150 mm, or from 40 to 100 mm. As further shown in FIG. 4, the exterior wire grid assembly 22 includes multiple vertical wires 22 b that are spaced apart a distance of D_(V). The vertical wire spacing D_(V) may typically range from 20 to 200 mm, for example, from 30 to 150 mm, or from 40 to 100 mm. The horizontal and vertical wire spacings D_(H) and D_(V) may be the same or different. The interior wire grid 23 of the first wire grid assembly 21, as well as the exterior and interior wire grids 26 and 27 of the second wire grid assembly 25, may have similar horizontal and vertical wire spacings D_(H) and D_(V) as those of the exterior wire grid 22.

As shown in FIG. 2, the reinforcing structure 20 has an overall thickness T_(R) that may typically range from 6.5 to 30 cm, for example, from 8 to 20 cm, or from 10 to 15 cm. The reinforced blast-resistant cementitious panel 10 has an overall panel thickness T_(P) that may typically range from 7 to 40 cm, for example, from 8 to 30 cm, or from 10 to 20 cm. While these ranges of the reinforcing structure thickness T_(R) and overall panel thickness T_(P) may be suitable for certain applications, it is to be understood that the thicknesses T_(R) and T_(P) may be varied depending upon the particular use of a panel and its desired blast-resistance properties.

The reinforced blast-resistant cementitious panels of the present invention may be provided in any desired heights and widths. In certain embodiments, the panels may have heights of from 1.5 to 10 m, for example, from 8 to 10 m, or from 2.5 to 7 m. In certain embodiments, the panels may have widths of from 1 to 5 m, for example, from 1.2 to 4 m, or from 1.4 to 3 m. While reinforced panels having substantially flat external surfaces are primarily described herein, it is to be understood that the reinforced panels of the present invention may include other shapes such as rounded or bent surfaces.

As shown in FIG. 2, the reinforcing structure 20 is embedded within the reinforced blast-resistant panel 10. The first wire grid assembly 21 is embedded a distance of D_(E) from the surface of the cementitious material 12 on one exterior face of the panel 10, while the second wire grid assembly 25 is embedded a distance of D_(E) from the surface of the cementitious material 12 on the opposing exterior face of the panel 10. The embedded distance D_(E) may typically range from 0.25 to 7.5 cm, for example, from 0.5 to 4 cm, or from 1.5 to 3 cm. While the embedded distance D_(E) shown in FIG. 2 is the same for both the first wire grid assembly 21 and the second wire grid assembly 25, it is to be understood that different embedded distances D_(E) may be used if desired.

FIG. 3 illustrates another embodiment of the present invention in which a reinforced blast-resistant cementitious panel 110 similar to the panel 10 shown in FIG. 2 is provided, with the addition of a layer of foam 116 contained within the panel 110. The reinforced blast-resistant cementitious panel 100 includes a first layer of cementitious material 112 and a second layer of cementitious material 114 sandwiching the foam core layer 116 therebetween. The layer of foam 116 may be made of any suitable material such as expanded polystyrene (EPS), extruded expanded polystyrene (XEPS), polyisocyanurate (PIMA), and the like. The thickness of the foam layer 116 may be adjusted as desired, and typically is less than or equal to the spacing distance S_(A) between the first and second wire grid assemblies 21 and 25. In accordance with the embodiment shown in FIG. 3, the use of the foam layer 116 may decrease the overall weight of the panel 110, and may improve the thermal insulation properties of the panel 110. The foam layer 116 could be replaced or combined with other types of lightweight and/or thermally insulating materials.

Although only two wire grid assemblies 21 and 25 are shown in the embodiments of FIGS. 1-3, additional wire grid assemblies and/or wire grids may be included as part of the reinforcing structure 20, or may be provided as a separate reinforcing structure within the blast-resistant panels. For example, a third wire grid assembly (not shown) similar to the first and second wire grid assemblies 21 and 25 may be located between the first and second wire grid assemblies 21 and 25 in a plane substantially parallel with the planes of the other assemblies. In this case, the third wire grid assembly may be attached to the connecting wires 30 by welding or any other suitable means.

In accordance with embodiments of the present invention, the cementitious material 12, 112 and 114 may comprise any known type of cementitious material such as Portland cement, pozzolanic cement, coal fly ash, ground granulated blast furnace slag, and the like. The cementitious material may comprise concrete, mortar, and other known types of materials. In certain embodiments, the cementitious material may include sand, stone, aggregate and other materials that are conventionally added to cement.

In certain embodiments, the cementitious material may be reinforced with fibrous materials. For example, the reinforcing fibers may include carbon fibers, aramid fibers, polymeric fibers, nylon fibers, polypropylene fibers, kevlar fibers, basalt fibers or any other known type of fibrous material. In certain embodiments, the fiber reinforcements are discontinuous and may have typical lengths of 1.25 to 15 cm, or from 2 to 10 cm. Alternatively, continuous fiber reinforcement materials may be used in place of, or in addition to, discontinuous fiber reinforcements. For example, continuous fibers may be provided in the form of woven fiber applied to the various components of the reinforcing structure 20, e.g., the continuous fibers may be wound around, or woven between, the various wires and/or reinforcing rods of the reinforcing structure 20. The amount of fiber reinforcements contained in the cementitious material may be varied as desired. For example, when discontinuous fibers are used, they may be present in an amount of from 0.05 to 2.5 weight percent, or from 0.1 to 2 weight percent, based on the total weight of the cementitious material. In certain embodiments, effective amounts of discontinuous fiber reinforcements may be less than 2 weight percent, or less than 1 weight percent, while providing sufficient blast-resistance properties.

In accordance with embodiments of the present invention, the reinforced panels have desirable compressive strengths for improved blast-resistance. For example, the reinforced panels may have compressive strengths of greater than 5,000 psi, for example, greater than 10,000 or 12,000 psi. In certain embodiments, the reinforced panels may have compressive strengths ranging from greater than 15,000 psi up to 20,000 psi, 24,000 psi, or higher.

The following examples are intended to illustrate various aspects of the invention, and are not intended to limit the scope of the invention. Two different types of foam core panels were tested with varying reinforcement ratios and panel thicknesses, and compared against the performance of a solid pre-cast concrete panel under similar test conditions. Experimental and analytical findings of the tests conducted on these panels are described below.

EXAMPLES

Three panel types were assessed as follows:

1. PSM HP 80: 8-inch thick Single Polystyrene High Performance structural wall panel consisting of a 4-inch thick EPS foam core straddled with 2-inch thick concrete on each face. Both concrete layers were reinforced with two layers of electro-welded 3 mm wire mesh having an approximate spacing of 2.8 inches.

2. PSM HP 160—RB#5: 11⅜-inch thick Single Polystyrene High Performance structural wall panel consisting of a 6-inch thick EPS foam core straddled with 2.5-inch thick concrete on each face. Each concrete layer was reinforced with the same two layers of electro-welded 3 mm wire mesh and #5 longitudinal reinforcing bars spaced 5 inches on center placed between the wire mesh reinforcement.

3. PC HP 80—RB#5: 6-inch thick Precast High Performance structural wall panel with electro-welded 3 mm wire mesh with an approximate spacing of 2.8 inches. #5 longitudinal rebar was placed between the wire mesh at 6 inches on center, which is slightly more than the 5-inch spacing for the PSM HP 160 panels. The concrete in this panel was reinforced with two different types of short and long synthetic fibers.

Table 1 summarizes the panel dimensions and properties used for the structural analysis.

TABLE 1 Panel Dimensions and Properties PSM HP 160 - PC HP 80 - Unit PSM HP 80 RB#5 RB#5 Panel Span (ft) 8 8 8 Width (ft) 4 4 4 t_(panel) (in) 8 11.375 6 t_(foam) (in) 4 6.375 — t_(conc) (in) 2 2.5 — f′_(c) (psi) 6,000 6,000 7,500¹   Wire Mesh x_(in) (in) 4.25 7.36   2.75 x_(out) (in) 5.812 8.94 4 A_(wire) (in²) 0.011 0.011   0.011 s_(wire) (in) 2.8 2.8  2.8 f_(y) (psi) 90,000 90,000 90,000    Rebar d_(1 (Loaded)) (in) — 1.62   1.44 d_(2 (Non-loaded)) (in) — 0.88 — A_(rebar) (in²) — 0.31   0.31 s_(rebar) (in) — 5.0  6.0 f_(y) (psi) — 60,000 60,000    ¹Concrete is reinforced with both short and long synthetic fibers.

Dynamic testing of the prepared panel test specimens was conducted at a shock tube facility. The shock tube is a compressed air-driven test apparatus that consists of two major sections, a driver section and an expansion section. Blast pressures are generated when an aluminum rupture disk placed between the two sections fails, due to pressure in the driver section. A shock wave then travels down the expansion section and loads the test specimen at the end of the expansion section. The driver was baffled to reduce the effects of reloading by the reflections that exist in the shock tube.

Each flat wall panel was mounted into a reaction structure that provided simple supports to the panel while allowing for reaction load measurement at the bottom support. In the test specimen installation, the specimen spans vertically 8 feet. Load cells are used to measure support reactions at the bottom of the specimen. The load cells provide the only lateral restraint of the bottom channel that supports the panel.

The test load was measured using a total of three dynamic pressure transducers located on each side wall and the floor of the shock tube adjacent to the target end of the shock tube. The load reported for each test is the average of the three gauges and represents the load applied to the wall specimen. Two quartz force ring dynamic load cells were used to measure panel end reactions. Initially, the dynamic mid-span displacements were monitored using a laser distance meter positioned at mid-span and offset from the panel. However, due to damage sustained to the gauge during the initial tests as a result of significant deflections, accelerometers were subsequently used and mounted to the panel at mid-span to capture the panel acceleration, which was differentiated to obtain the mid-span displacement. High speed video was focused on a mid-span scale background to confirm the active measured panel displacement data.

Pre-test analytical models were derived for each panel type based on single-degree-of-freedom (SDOF) methodology in order to predict the flexural response of each panel prior to testing. Initial estimates of the first test load were selected to obtain a low to moderate damage response. After each test was conducted, the experimental response was compared to the predicted response and the model was then adjusted to represent the observed response. The revised model was then used to determine the magnitude of blast loads for subsequent tests with increasing levels of damage. The objective of this process was to obtain experimental results for varying pressure and impulse combinations in order to define acceptable response limits with respect to peak applied pressure and impulse.

Table 2 presents a summary of the test program providing the panel type, specimen number, peak pressure, impulse, and test duration for each test. The resulting panel response is reported using: (i) Δ_(peak)=peak mid-span displacement with corresponding time of occurrence; (ii) R_(peak)=peak support reaction with corresponding time of occurrence; (iii) S_(upport)=support rotation based of peak displacement; and (iv) response of panel in accordance with ASCE damage response criteria based on measured deflection. The observed damage was qualitatively less than what was designated by ASCE response criteria. For example, Test 3 for the PSM HP 80 Panel II, would be characterized as failure whereas the panel achieved a maximum deflection and survived the applied blast loading without failure.

TABLE 2 Summary of Experimental Results Panel Pressure Impulse Δ_(peak) @ t R_(peak) @ t θ_(support) ASCE Type Panel Test (psi) (psi-ms) (in) (ms) (lb) (ms) (deg) Response PSM HP I  1 3.9 160 2.6 73 9,133 29 3.1 High 80  2¹ 4.8 197 N/A N/A 9,000 45 N/A Failure II  3 5.1 210 5.07 78 7,043 18 5.9 Failure PSM HP I 16 4.8 222 0.36 31 11,295 33 0.4 Low 160 - 17¹ 10.7 508 2.68 55 18,370 24 3.3 High RB#5 II 18 15.3 562 14.2² 90 28,853 35 19.5 Failure III 19 21.9 445 15.7² 95 31,210 13 16.3 Failure IV 20 12.3 335 3.19 58 20,689 52 3.6 High PC HP 80 - I  7 5.2 224 0.38 23 19,890 23 0.5 Low RB#5  8¹ 11.8 760 0.9 22 39,475 24 1.1 Med  9¹ 16.6 1,528 1.5 22 54,180 23 1.8 Med II 10 23.6 496 2.5 30 66,316 25 3.0 High ¹Retest of previous panel identified using italics ²Panel sustained a direct shear failure at reactions, resulting in the panel unseating from the support and increasing the recorded deflections.

Three tests were conducted on two PSM HP80 panel specimens ranging from 3.9 psi to 5.1 psi with impulses of 160 psi-ms to 210 psi-ms, respectively. All tests exhibited a typical flexural response with horizontal cracks evenly distributed along the height of the non-loaded tension face as shown in FIGS. 5A-5C. Test 1 resulted in a peak deflection of 2.6 inches at 73 ms, corresponding to a High response criterion as defined by ASCE. Test 2 was a re-test of Specimen I and resulted in clean break of the panel at a pressure of 4.8 psi where fracture of the mesh wire was the primary failure mechanism with minimal concrete spalling on the compression face. As this was a re-test, the panel had an initial level of plastic strain in the mesh and cracked concrete, therefore, the test was not a true representation of an undamaged panel response. Test 3 subjected a new test panel to a slightly higher blast load resulting in a peak deflection of 5.0 inches. This response corresponds to a support rotation of 5.9 degrees and exceeds a High response according to ASCE criteria (Failure), even though the panel did not fail and fall out of the testing frame.

Five tests were conducted on four PSM HP160—RB#5 panel specimens ranging from 4.2 psi to 21.9 psi with impulses of 222 psi-ms to 445 psi-ms, respectively. All tests primarily responded in flexural with the formation of single hinge at mid-span, coupled with concrete spalling on the compression face as shown in FIGS. 6A-6C and 7A-7F. The observed concrete spalling can be attributed to the high levels of longitudinal reinforcement used in each concrete panel.

Test 16 applied a blast load similar to Test 3 (PSM HP80) in order to obtain a direct correlation between the two panel types. The panel responded elastically to the blast load with a peak deflection of 0.36 inches and had no residual surface cracking, thus exhibiting a significant increase in blast resistance in comparison to the PSM HP80 panels. Test 17 was a retest of Specimen I with in increased blast load of 10.7 psi resulting in a peak deflection of 2.8 inches as shown in FIGS. 6A-6C. In addition to small residual flexural cracking, damage was observed at the bottom right support, which was attributed to the mesh being wrapped around the outside edges of the pane that caused the cover concrete to spall due to rotation at the supports. Initial signs of concrete spallation at mid-span were also observed on the loaded face.

Tests 18 and 19 subjected new test panels to peak applied blast pressures of 15.3 psi and 21.9 psi, respectively, and both resulted in heavy damage approaching incipient failure. In addition to the primary flexural hinge formed at mid-span, each panel also sustained significant shear failure at their supports causing the panels to unseat from the supports as shown in FIGS. 7A-7E. Tests 18 and 19 indicated permanent deformations of 8.75 inches and 6.5 inches at midspan, respectively; with an overall reduction in panel thickness of 1.63 inches and 0.625 inches due to compression of the foam core, respectively. Test 20 subjected a new test panel to a reduced blast pressure of 12.1 psi resulting in a peak deflection of 3 inches. Signs of incipient concrete spall were observed on the right side of the loaded face as shown in FIG. 7F. Post-test measurements indicated a permanent displacement of 3.6 inches at mid-span, and an overall reduction in panel thickness of 0.375 inches due to foam crushing.

Four tests were conducted on two PC HP80—-RB#5 panel specimens ranging from 5.2 psi to 23.6 psi with impulses of 224 psi-ms to 496 psi-ms, respectively. All tests exhibited a typical flexural response with horizontal cracks evenly distributed along the height of the non-loaded face as shown in FIGS. 8A-8C. Tests 7 through 9 were conducted on Specimen I with increasing blast pressures, in which Test 9 had a peak applied blast pressure of 16.6 psi and impulse of 1,528 psi-ms. This resulted in a peak deflection of 1.5 inches and corresponds to a Medium response criterion in accordance with ASCE. Test 10 subjected a new panel to a peak applied blast pressure of 23.6 psi, resulting in peak deflection of 2.5 inches. Although classified as a High response based on a measured 3 degree support rotation, the panel showed no sign of concrete spall or any other severe damage, which was attributed to the high level of reinforcement in combination with the fiber reinforced concrete.

The structural performance of the PSM HP foam core panels is largely dependent on the interaction between the two concrete layers and the corresponding level of composite behavior. FIGS. 9A-9C illustrate the difference between full-composite and non-composite behavior. Full-composite action assumes that the dowel rods and foam core connecting the two concrete layers have sufficient strength and stiffness to transfer in-plane shear flow between the layers such that the entire section remains plane. Conversely, non-composite action assumes that no in-plane shear flow is transferred between layers such that the panel response can be defined as the superposition of the each sub-panels response.

The actual response of these panels will be a partially composite response that is a function of the ratio between the shear flow and flexural capacities; where the shear flow capacity is provided by the wire dowels and foam core connecting the concrete layers, and the flexural bending capacity is provided by the longitudinal reinforcement. A simplified modeling approach was initially adopted for the analytical modeling of these panels using a Single-Degree-of-Freedom (SDOF) model to evaluate the nonlinear dynamic response of each panel when subjected to the averaged blast pressure-time history from each test. The following section describes the SDOF model used for each panel, and compares the SDOF model results to the experimental results. A summary of the peak displacements (Δ_(peak)) and peak reactions (R_(peak)) for the experimental and SDOF analytical results are provided in Table 3.

PSM HP 80 Panel: The flexural capacity is provided by the longitudinal mesh only and was relatively balanced with the shear flow capacity provided by the wire dowel connectors and foam core. Therefore, the blast capacity was better captured by the full-composite model; however, because the wire dowel connectors are embedded in the foam, the stiffness was better captured using the non-composite model. The SDOF model results were within 16% agreement with the experimental results, which was reasonable considering the level of complexity involved with modeling these panels.

PSM HP 160—RB#5: In comparison to the PSM HP 80 Panels, the flexural capacity of the PSM HP 160 Panels is significantly increased with the inclusion of the #5 rebar; however, the shear flow resistance provided by the wire dowel connectors remained unchanged. Post-experimental analysis concluded that the blast capacity of the panel was on the order of halfway between the full- and non-composite model due to the fractional amount of shear flow transferred by the dowel wires and foam core. This approximation resulted in comparisons within 43% of the experimental results regarding the measured reactions; however, the observed displacements had a much greater deviation. The deflections measured for Tests 18 and 19 are not representative of the flexural response anticipated, due to the shear failure sustained at the panel supports.

PC HP 80—RB#5: The SDOF model for this panel was derived using a moment-curvature relationship of the doubly reinforced concrete section, assuming standard flexural theory. Standard concrete constitutive material models were assumed for this model as no material test data was available for the fiber reinforced concrete used in the construction of these panels. When compared to the experimental results, the analytical model consistently over predicted the deflections up to 13%, and under predicted the reactions no more than 38%. One possible explanation for these differences may be due to the additional stiffness provided by the concrete fibers under high strain-rate effects.

TABLE 3 SDOF Results Compared to Test Measurements Blast Results SDOF Analytical Model Comparison   Test   Pressure   Δ_(peak) (in)   @ t (ms)   R_(peak) (lb)   @ t (ms)   Δ_(peak) (in)   @ t (ms)   R_(peak) (lb)   @ t (ms) $\frac{\Delta_{model}}{\Delta_{\exp}}$ $\frac{R_{model}}{R_{\exp}}$ PSM HP 80 Panel  1 3.9  2.6 73  9,133 29 2.2 60  7,280 24 0.85 0.80  2 ¹ 4.81 N/A² N/A²  9,000 45 3.5 70  7,603 21 N/A 0.84  3 5.1  5.07 78  7,043 18 4.6 72  7,741 22 0.91 1.10 PSM HP 160-RB#5 16 4.8  0.36 31 11,295 33 0.8 32 13,317 32 2.22 1.18 17 ¹ 10.7  2.68 55 18,370 24 1.7 33 26,312 25 0.63 1.43 18 15.3 14.2³ 90 28,853 35 3.8 45 27,325 17 0.27³ 0.95 19 21.9 15.7³ 95 31,210 13 4.7 37 30,920 13 0.30³ 0.99 20 12.3  3.19 58 20,689 52 1.93 33 25,298 20 0.61 1.22 PC HP 80-RB#5 Panel  7 5.2  0.38 23 19,890 23 0.43 24 14,976 24 1.13 0.75  8 ¹ 11.8  0.9 22 39,475 24 0.95 24 32,855 24 1.06 0.83  9 ¹ 16.6  1.5 22 54,180 23 1.65 30 39,536 30 1.10 0.73 10 23.6  2.5 30 66,316 25 2.68 30 40,920 16 1.07 0.62 ¹Italics represent retest of previous panel; concrete is already cracked and may not be representative of untested specimen. ²Panel failed so peak deflection is not available. ³Panel sustained a direct shear failure at reactions, resulting in the panel unseating from the support and increasing the recorded deflections.

In order to further investigate the level of composite interaction between the two layers of reinforced concrete outer layers in the PSM HP 80 and PSM HP 160—RB#5 panels, DYNA-3D Finite Element Models (FEM) were developed for these two panel types. Hexahedral (solid brick) elements were used to represent the concrete and foam core, while structural beam elements were used to model the reinforcing steel. Strength Increase Factor (SIF) and Dynamic Increase Factor (DIF) were applied to all material strengths per guidelines in the Unified Facilities Criteria (UFC). Concrete element size was generally 0.50×0.50×0.40 inch in the PSM HP 160—RB#5 model, and 0.50×0.50×0.30 inch in the PSM HP 80 model. The reinforcing steel beam elements shared nodes with the concrete and foam mesh (i.e., no provision for bond-slip between rebar and concrete). Physical properties for the foam were not available, so typical properties were assumed. A contact surface was applied over the top and bottom 2 inches of the panels providing a free span of 96 inches between simple supports. The panels were not allowed to develop vertical in-plane deflection resistance at the supports such that no arching or membrane behavior could occur.

The concrete was modeled using the Applied Engineering Cap Model with Three Invariants (AEC-31, implemented into DYNA-3D). The AEC-3I models material plasticity with a unified shear yield and cap surface that can harden from an initial yield position to ultimate strength and then soften to residual surfaces under continued loading. Decohesion algorithms model the formation of crack planes with defined orientation and controlled growth based on material fracture energy, which is specified as a user input. Crack growth is represented with decohesion strain components resulting in anisotropic material behavior. Orientation of crack plane initiation is based on principal-stress Rankine criteria that are characteristic of brittle-fracture materials.

Comparisons of the peak displacements (Δ_(peak)) and peak reactions (R_(peak)) for the experimental and DYNA-3D FEM results are provided in Table 4. The peak deflections represent complicated interactions of concrete damage (cracking and compressive shear), foam crushing, and flexural/buckling behavior of the through-thickness shear steel. An example of the resulting crack pattern at the moment of peak deformation from Test 17 is shown in FIGS. 10A and 10B. The calculations for Tests 18 and 19 terminated before peak deflections were reached due to excessive element distortions in the foam.

FIGS. 11A-11F provide a comparison of the SDOF and DYNA-3D analytical models for the displacement and dynamic reaction time history results of Tests 3, 16, and 20. Overall, the DYNA-3D FEM results were in good agreement with the experimental results and provided a more accurate representation of the observed response and crack pattern formations, particularly for the PSM HP 160—RB#5 panels.

TABLE 4 DYNA-3D Results Compared with Test Measurements Blast Results DYNA-3D Analytical Model Comparison   Test   Pressure   Δ_(peak) (in)   @ t (ms)   R_(peak) (lb)   @ t (ms)   Δ_(peak) (in)   @ t (ms)   R_(peak) (lb)   @ t (ms) $\frac{\Delta_{model}}{\Delta_{\exp}}$ $\frac{R_{model}}{R_{\exp}}$ PSM HP 80 Panel  1 3.9  2.6 73  9,133 29 1.43 63  7,772 20 0.55 0.85  3 5.1  5.07 78  7,043 18 3.08 79  8,520 22 0.61 1.21 PSM HP 160-RB#5 16 4.8  0.36 31 11,295 33 0.37 48 10,969 22 1.03 0.97 17 ¹ 10.7  2.68 55 18,370 24 3.96 74 18,187 42 1.48 0.99 18 15.3 14.2² 90 28,853 35 >6.3³ N/A 19,591 25 N/A 0.68 19 21.9 15.7² 95 31,210 13 >6.1⁴ N/A 23,477 16 N/A 0.75 20 12.3  3.19 58 20,689 52 3.44 54.5 17,385 34 1.08 0.84 ¹Italics represent retest of previous panel; concrete is already cracked and may not be representative of virgin specimen. ²Panel sustained a direct shear failure at reactions, resulting in the panel unseating from the support and increasing the recorded deflections. ³Continued deflection at termination of calculation at 54.5 ms due to element distortion ⁴Continued deflection at termination of calculation at 45 ms due to element distortion

The test program provided test data showing reasonable blast capacity of the reinforced foam core panels when compared to the tested solid core panels, although significantly less blast damage was observed for the solid panels under similar blast loading. For example, the PSM HP 80 Panel II (Test 3) showed moderate to heavy damage when subjected to 5.1 psi/210 psi-ms compared to the PC HP 80—RB#5, Panel I (Test 7) that showed light damage when subjected to 5.2 psi/224 psi-ms. Note that the PSM HP 80 panel had much less flexural reinforcement and was a third lighter compared to the PC HP 80—RB#5 panel. Another example was the PSM HP 160—RB#5 Panel III (Test 19) that showed heavy damage when subjected to 21.9 psi/445 psi-ms and the PC HP 80—RB#5 Panel V (Test)) that show moderate damage when subjected to 23.6 psi/496 psi-ms. Although both panels had similar reinforcement, the PSM HP 160—RB#5 panel was almost 20 lighter than the PC HP 80—RB#5 panel.

Both SDOF and FEA models of the panel responses produced reasonable comparisons with the test results, with the FEA models showing better agreement. These models were able to capture the failure mechanisms associated with the foam core panels with respect to diagonal shear of the concrete layers as well as the in-plane shear capacity at the foam-concrete surface. These models can be reasonably be used to better define the response limits of specific foam-core designs ranging in concrete and foam thickness, and reinforcement combinations.

The reinforced cementitious panels of the present invention provide several advantages. Lighter inexpensive cladding building systems can be implemented into building blast hardened construction. A cost-effective reinforced concrete foam core panel may take advantage of increased section depth with minimal effect on panel weight. The reinforced panels may use high-strength concrete specifically designed for in-situ applications either before or after installation of the panel units, and the connecting wires provide shear reinforcement. The present system may improve constructability for applications where access is difficult, as well as providing enhanced thermal characteristics.

Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims. 

What is claimed is:
 1. A reinforcing structure for a blast-resistant cementitious panel comprising: a first wire grid assembly comprising an exterior wire grid and an interior wire grid spaced from the exterior wire grid; a second wire grid assembly spaced from the first wire grid assembly comprising at least an exterior wire grid; and a plurality of connecting wires attached to the wire grids of the first and second wire grid assemblies.
 2. The reinforcing structure of claim 1, wherein the first wire grid assembly is substantially planar, the second wire grid assembly is substantially planar, and the planes of the first and second wire grid assemblies are substantially parallel with each other.
 3. The reinforcing structure of claim 2, wherein the second wire grid assembly further comprises an interior wire grid, and each of the plurality of connecting wires is attached to the exterior and interior wire grids of the first wire grid assembly and the exterior and interior wire grids of the second wire grid assembly by welding.
 4. The reinforcing structure of claim 3, wherein the plurality of connecting wires are substantially perpendicular to the planes of the first and second wire grid assemblies.
 5. The reinforcing structure of claim 3, further comprising a first plurality of reinforcing rods located between the exterior and interior wire grids of the first wire grid assembly.
 6. The reinforcing structure of claim 5, further comprising a second plurality of reinforcing rods located between the exterior and interior wire grids of the second wire grid assembly.
 7. The reinforcing structure of claim 6, wherein the exterior and interior grids of the first and second wire grid assemblies comprise wires having diameters of from 1 to 7 mm, the plurality of connecting wires have diameters of from 1 to 7 mm, and the first and second pluralities of reinforcing rods have diameters of from 8 to 40 mm.
 8. The reinforcing structure of claim 6, wherein the first and second wire grid assemblies are spaced apart from 1 to 20 mm.
 9. The reinforcing structure of claim 6, wherein the exterior and interior wire grids of the first wire grid assembly are spaced apart substantially the same distance as a diameter of the first plurality of reinforcing rods located therebetween, and the exterior and interior grids of the second wire grid assembly are spaced apart substantially the same distance as a diameter of the second plurality of reinforcing rods located therebetween.
 10. The reinforcing structure of claim 6, wherein the first and second pluralities of reinforcing rods are held in position by frictional engagement with the wire grids.
 11. The reinforcing structure of claim 6, wherein the first and second pluralities of reinforcing rods are attached to the wire grids by welding and/or mechanical fasteners.
 12. The reinforcing structure of claim 6, wherein the exterior and interior wire grids of the first wire grid assembly are spaced apart from 8 to 40 mm, and the exterior and interior wire grids of the second wire grid assembly are spaced apart from 8 to 40 mm.
 13. The reinforcing structure of claim 6, wherein the exterior and interior wire grids of the first and second wire grid assemblies comprise perpendicular wires forming substantially rectangular openings with heights of from 20 to 200 mm and widths of from 20 to 200 mm.
 14. The reinforcing structure of claim 6, wherein the first and second pluralities of reinforcing bars are substantially parallel with each other.
 15. The reinforcing structure of claim 6, wherein the structure has an overall thickness of from 6.5 to 30 cm, a height of from 1.5 to 10 m, and a width of from 1 to 5 m.
 16. The reinforcing structure of claim 6, wherein the first and second wire grid assemblies, the plurality of connecting wires, and the first and second pluralities of reinforcing rods comprise steel.
 17. The reinforcing structure of claim 1, further comprising a third wire grid assembly between the first and second wire grid assemblies.
 18. The reinforcing structure of claim 1, further comprising a layer of foam between the first wire grid assembly and the second wire grid assembly.
 19. The reinforcing structure of claim 1, wherein the reinforcing structure is pre-formed prior to addition of a cementitious material to the structure.
 20. A reinforced blast-resistant cementitious panel comprising: a cementitious material; and a reinforcing structure in the cementitious material, wherein the reinforcing structure comprises: a first wire grid assembly comprising an exterior wire grid and an interior wire grid spaced from the exterior wire grid; a second wire grid assembly spaced from the first wire grid assembly comprising at least an exterior wire grid; and a plurality of connecting wires attached to the wire grids of the first and second wire grid assemblies.
 21. The reinforced cementitious panel of claim 20, wherein the first wire grid assembly is embedded in the cementitious material a distance of from 0.25 to 7.5 cm from a first planar surface of the panel, and the second wire grid assembly is embedded in the cementitious material a distance of from 0.25 to 7.5 cm from a second planar surface of the panel opposite from the first planar surface of the panel.
 22. The reinforced cementitious panel of claim 20, wherein the cementitious material comprises Portland cement, pozzolanic cement, coal fly ash and/or ground granulated blast furnace slag.
 23. The reinforced cementitious panel of claim 20, wherein the cementitious material comprises concrete.
 24. The reinforced cementitious panel of claim 20, wherein the cementitious material includes reinforcing fibers dispersed therein comprising carbon fibers, aramid fibers, polymeric fibers, nylon fibers, polypropylene fibers, kevlar fibers and/or basalt fibers.
 25. The reinforced cementitious panel of claim 24, wherein the reinforcing fibers comprise nylon.
 26. The reinforced cementitious panel of claim 24, wherein the reinforcing fibers have lengths of from 1.25 to 15 cm and are present in the cementitious material in an amount of from 0.05 to 2.5 weight percent based on the total weight of the cementitious material.
 27. The reinforced cementitious panel of claim 20, wherein the reinforced panel has a compressive strength of greater than 10,000 psi.
 28. The reinforced cementitious panel of claim 20, further comprising a layer of foam embedded in the cementitious material between the first wire grid assembly and the second wire grid assembly.
 29. A method of making a reinforced blast-resistant cementitious panel comprising introducing a flowable cementitious material to a pre-formed reinforcing structure and allowing the cementitious material to cure, wherein the reinforcing structure comprises: a first wire grid assembly comprising an exterior wire grid and an interior wire grid spaced from the exterior wire grid; a second wire grid assembly spaced from the first wire grid assembly comprising at least an exterior wire grid; and a plurality of connecting wires attached to the wire grids of the first and second wire grid assemblies. 